It contains the official legal advice given to the government on the Withdrawal Agreement - in particular, it is said to include revealing details about the permanency of the Irish border backstop.

MPs voted last month to force its disclosure (through an arcane procedure which ministers also tried to resist). When MPs voted, the outcome was theoretically binding on ministers.

But yesterday, the government instead gave MPs a longer explanatory document - not the raw text they had demanded.

Contempt vote

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Attorney General Geoffrey Cox delivered a robust defence of the government's position, arguing that the secret document contains information protected by lawyer-client privilege and its contents could be used to help the EU in any future negotiations.

But it was not enough for opposition parties. Labour has tabled a motion backed by opposition parties at Westminster, including the government's own bedfellows, the DUP.

It calls for the immediate publication of the full advice and will - if passed - find the government in contempt of Parliament for behaving in this manner.

The debate is likely to start at 12.45 and will be led by Shadow Brexit Secretary Keir Starmer.

The government has not yet said which minister will come to Parliament to defend it.

Penalties

There will be a vote after a few hours of debate - MPs will be asked to pass judgement on whether ministers are in contempt. Many will relish this vote - which could be the only one of its kind in their parliamentary careers.

It's not certain what will happen if the government is found in contempt of the House. Historically, MPs found in contempt would be imprisoned by the Serjeant-at-Arms.